Mesa semiconductor device and method of manufacturing the same

ABSTRACT

The invention provides a mesa semiconductor device and a method of manufacturing the same which minimize the manufacturing cost and prevents contamination and physical damage of the device. An N− type semiconductor layer is formed on a front surface of a semiconductor substrate, and a P type semiconductor layer is formed thereon. An anode electrode is further formed on the P type semiconductor layer so as to be connected to the P type semiconductor layer, and a mesa groove is formed from the front surface of the P type semiconductor layer deeper than the N− type semiconductor layer so as to surround the anode electrode. Then, a second insulation film is formed from inside the mesa groove onto the end portion of the anode electrode. The second insulation film is made of an organic insulator such as polyimide type resin or the like. The lamination body made of the semiconductor substrate and the layers laminated thereon is then diced along a scribe line.

CROSS-REFERENCE OF THE INVENTION

This application claims priority from Japanese Patent Application No.2007-330329, the content of which is incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a mesa semiconductor device having a mesagroove and a method of manufacturing the same.

2. Description of the Related Art

A high power mesa diode is conventionally known as one of mesasemiconductor devices. A conventional mesa diode will be describedreferring to FIGS. 7 and 8. FIG. 7 is a schematic plan view of asemiconductor wafer where a plurality of conventional mesa diodes isdisposed in a matrix. FIG. 8 is a cross-sectional view of FIG. 7 alongline X-X, showing a state after dicing along a scribe line DL.

An N− type semiconductor layer 11 is formed on a front surface of an N+type semiconductor substrate 10. A P type semiconductor layer 12 isformed on the front surface of the N− type semiconductor layer 11, and afirst insulation film 23 is formed on the P type semiconductor layer 12.An anode electrode 14 electrically connected to the P type semiconductorlayer 12 is further formed. A cathode electrode 15 is formed on the backsurface of the semiconductor substrate 10.

A mesa groove 26 is formed from the front surface of the P typesemiconductor layer 12 to the N+ type semiconductor substrate 10. Themesa groove 26 is formed deeper than the N− type semiconductor layer 11,of which the bottom is located in the N+ type semiconductor substrate10. A second insulation film 47 fills the mesa groove 26 so as to coverthe sidewall thereof including the PN junction JC of the N− typesemiconductor layer 11 and the P type semiconductor layer 12 which arein contact. The mesa diode is surrounded by this mesa groove 26, forminga mesa structure. The scribe line DL of this mesa diode surrounds themesa groove 26 on its outside.

A mesa semiconductor device is described in Japanese Patent ApplicationPublication No. 2003-347306, for example.

In the conventional example described above, however, since the anodeelectrode 14 is exposed, water or the like enters the mesa diode from agap between the anode electrode 14 and the first insulation film 23 tocontaminate the mesa diode. Furthermore, the mesa diode is easilydamaged physically.

For solving this problem, it is conceivable that a passivation film isfurther formed from the mesa groove 26 onto the end portion of the anodeelectrode 14 in addition to the second insulation film 47. In this case,however, the manufacturing process becomes complex by adding a processof forming the passivation film, and the manufacturing cost increases.

SUMMARY OF THE INVENTION

The invention provides a mesa semiconductor device that includes asemiconductor substrate having a first semiconductor layer of a firstgeneral conductive type and a second semiconductor layer of a secondgeneral conductive type formed on the front surface of the firstsemiconductor layer so as to have a PN junction between the first andsecond semiconductor layers, a first insulation film covering the frontsurface of the second semiconductor layer and having an opening, anelectrode formed on the front surface of the second semiconductor layerso as to be in contact with the second semiconductor layer through theopening of the first insulation film, a mesa groove formed in thesemiconductor substrate from the front surface of the secondsemiconductor layer to define a mesa on top of which the electrode isdisposed, and a second insulation film filling the mesa groove andextending onto the front surface of the second semiconductor layer so asto cover the first insulation film and the end portion of the electrode.

The invention also provides a method of manufacturing a mesasemiconductor device. The method includes providing a semiconductorsubstrate having a first semiconductor layer of a first generalconductive type and a second semiconductor layer of a second generalconductive type formed on the front surface of the first semiconductorlayer so as to have a PN junction between the first and secondsemiconductor layers, forming a first insulation film on the frontsurface of the second semiconductor layer so as to have an openingexposing the front surface of the second semiconductor layer, forming anelectrode on the exposed front surface of the second semiconductor layerthrough the opening of the first insulation film, forming a mesa groovein the semiconductor substrate from the front surface of the secondsemiconductor layer to define a mesa on top of which the electrode isdisposed, and forming a second insulation film so as to fill the mesagroove and extend onto the front surface of the second semiconductorlayer to cover the first insulation film and the end portion of theelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 6 are cross-sectional views showing a mesa diode and a methodof manufacturing the same of an embodiment of the invention.

FIG. 7 is a plan view of a conventional mesa diode.

FIG. 8 is a cross-sectional view of the conventional mesa diode.

DETAILED DESCRIPTION OF THE INVENTION

A mesa semiconductor device of an embodiment of this invention and amethod of manufacturing that device will be described using a mesa diodeas an example. FIGS. 1 to 6 are cross-sectional views showing the mesadiode and the method of manufacturing the device and correspond to across-section of FIG. 7 along line X-X. In FIGS. 1 to 6, the samereference numerals are given to the same components as those shown inFIG. 7.

The method of manufacturing the mesa diode described below is conductedto a semiconductor wafer where a plurality of mesa diodes is to bedisposed in a matrix as shown in FIG. 7. In FIGS. 1 to 6, forconvenience of the description, one of plural mesa diodes is mainlyshown and two mesa diodes adjacent to this are partially shown.

As shown in FIG. 1, an N+ type semiconductor substrate 10 (e.g. asilicon single crystal substrate) where a high concentration of N typeimpurity such as, for example, phosphorus is diffused is provided. A lowconcentration N type semiconductor layer, i.e., an N− type semiconductorlayer 11 is formed on the front surface of the semiconductor substrate10 by, for example, epitaxially growing a semiconductor layer. Insteadof this, the N− type semiconductor layer 11 may be an impurity diffusedregion formed by diffusing an impurity in the front surface of thesemiconductor substrate 10. Then, a P type semiconductor layer 12 isformed on the front surface of the N− type semiconductor layer 11 by,for example, diffusing a P type impurity such as boron. By this process,a PN junction JC is formed at the interface of the N− type semiconductorlayer 11 and the P type semiconductor layer 12. In the structuredescribed above, the total thickness of the semiconductor substrate 10,the N− type semiconductor layer 11 and the P type semiconductor layer 12is about 200 μm, for example.

It is noted that conductivity types such as N+, N and N− belong in onegeneral conductivity type, and conductivity types such as P+, P and P−belong in another general conductivity type.

Then, as shown in FIG. 2, a first insulation film 23 such as a siliconoxide film is formed on the front surface of the P type semiconductorlayer 12 by, for example, a thermal oxidation method or a CVD method.The first insulation film 23 is then partially etched using a mask toprovide the first insulation film 23 with a first opening 23A and asecond opening 23B exposing the P type semiconductor layer 12 partially.The first opening 23A corresponds to the active region of the mesadiode, and the second opening 23B corresponds to a scribe line regionwhere a scribe line DL extends.

Then, an anode electrode 14 connected to the P type semiconductor layer12 through the first opening 23A of the first insulation film 23 isformed. The anode electrode 14 is made of a conductive material such asaluminum and formed by a sputtering method, a vapor deposition method orthe like. A cathode electrode 15 made of a conductive material such asaluminum is formed on the back surface of the semiconductor substrate 10by the same method as the method of forming the anode electrode 14.

Then, as shown in FIG. 3, a resist layer PR covering the firstinsulation film 23 is formed. The resist layer PR has an opening PRAcorresponding to a region for forming a mesa groove 26 which will bedescribed below. Then, the first insulation film 23 exposed in theopening PRA is etched and removed using this resist layer PR as a maskto provide the first insulation film 23 with a third opening 23C. Then,the P type semiconductor layer 12, the N− type semiconductor layer 11and the semiconductor substrate 10 are etched in a region reaching themiddle of the semiconductor substrate 10 in the thickness directionusing the resist layer PR as a mask, thereby forming the mesa groove 26surrounding the active region of the mesa diode. In this etching, themesa groove 26 having a high aspect ratio is obtained by using the Boschprocess in which isotropic dry-etching and formation of a protectionfilm are repeated, anisotropic dry-etching under extremely low pressure,or the like. The bottom of the mesa groove 26 is located deeper than theN− type semiconductor layer 11 and reaches the inside of thesemiconductor substrate 10. The total depth is preferably about 100 μm.In other words, the mesa groove 26 reaches about half the depth of thestack of the semiconductor substrate 10 and the semiconductor layers 11and 12. The width of the mesa groove 26 is about 10 μm, for example.

The mesa diode having such a mesa groove 26 has a high breakdown voltagewhen a reverse bias is applied, i.e., when a high voltage is appliedfrom the cathode electrode 15 to the anode electrode 14 and a reversebias is applied to the PN junction JC.

Then, the resist layer PR is removed as shown in FIG. 4, and then asshown in FIG. 5, a second insulation film 27 is formed continuouslycovering the P type semiconductor layer 12 exposed in the second opening23B of the first insulation film 23 (i.e., in the scribe line regionalong the scribe line DL), the first insulation film 23, the inside ofthe mesa groove 26 and the end portion of the anode electrode 14. Thatis, the second insulation film 27 has an opening 27A exposing the anodeelectrode 14 except its end portion. A part of the second insulationfilm 27 extending on the end portion of the anode electrode 14 is atleast about 50 μm from the end of the anode electrode 14, for example.

The second insulation film 27 is made of an organic insulator having,during its application to the substrate 10, a viscosity such that itfills the second opening 23B and the mesa groove 26 and continuouslyextends onto the anode electrode 14 therefrom. For example, a viscosityof 100 to 150 Pa·s during the application is good enough to achieve anappropriate coverage. After this application, the second insulation film27 is cured, i.e., becomes more rigid. The second insulation film 27contains polyimide type resin or epoxy type resin, for example.Alternatively, instead of the organic insulator described above, glasspaste made by mixing lead or zinc type glass powder and resin may beused as the second insulation film 27 as long as it has the sameviscosity as described above. The second insulation film 27 is formed bya screen printing method, a dispensing method or a spin coating method,for example, and patterned by a photolithography process or the likeaccording to needs.

Then, as shown in FIG. 6, the lamination body made of the semiconductorsubstrate 10 and the layers laminated thereon is diced along the scribeline DL extending in the second opening 23B and separated into aplurality of mesa diodes.

Since the mesa diode completed in this manner is covered by the secondinsulation film 27 from inside the mesa groove 26 onto the end portionof the anode electrode 14 continuously, contamination of the inside ofthe mesa diode due to water entering it or the like and physical damageof the mesa diode are prevented without an additional passivation film.This eliminates a process of forming the additional passivation film,thereby minimizing the manufacturing cost.

Furthermore, since the second insulation film 27 fills the mesa groove26 deeper than the N− type semiconductor layer 11, reaching the insideof the semiconductor substrate 10, it functions as a guard ring forpreventing water entering the active region of the mesa diode.

The invention is not limited to the embodiment described above andmodifications are possible within the scope of the invention. Forexample, in the above embodiment, the first insulation film 23 is notnecessarily formed in the region on the outside of the mesa groove 26.In this case, in the region on the outside of the mesa groove 26, thesecond insulation film 27 is formed on the front surface of the P typesemiconductor layer 12.

Furthermore, the N+ type semiconductor substrate 10, the N− typesemiconductor layer 11 and the P type semiconductor layer 12 in theembodiment described above may be of opposite conductive types,respectively. Furthermore, the P type semiconductor layer may be formeddirectly on the N type semiconductor substrate. Furthermore, althoughthe description is given using the mesa diode as an example in theembodiment described above, the invention may be applied to other mesasemiconductor devices. For example, the invention may also be applied toa mesa bipolar transistor, a mesa MOSFET, a mesa IGBT, a mesa thyristoror the like. In the case of the mesa bipolar transistor, for example, anNPN type bipolar transistor structure is obtained by further providingan N type semiconductor layer on the front surface of the P typesemiconductor layer 12. Thus, with this structure, the second insulationfilm extending from the mesa groove covers a part of the electrode,thereby preventing contamination of the mesa semiconductor device due towater entering it or the like and physical damage of the mesasemiconductor device without an additional passivation film.Furthermore, this eliminates a process of forming the additionalpassivation film, thereby reducing the manufacturing cost.

1. A mesa semiconductor device comprising: a semiconductor substratecomprising a first semiconductor layer of a first general conductivetype and a second semiconductor layer of a second general conductivetype formed on a front surface of the first semiconductor layer so as tohave a PN junction between the first and second semiconductor layers; afirst insulation film covering a front surface of the secondsemiconductor layer and having an opening; an electrode formed on thefront surface of the second semiconductor layer so as to be in contactwith the second semiconductor layer through the opening of the firstinsulation film; a mesa groove formed in the semiconductor substratefrom the front surface of the second semiconductor layer to define amesa on top of which the electrode is disposed; and a second insulationfilm filling the mesa groove and extending onto the front surface of thesecond semiconductor layer so as to cover the first insulation film andan end portion of the electrode.
 2. The mesa semiconductor device ofclaim 1, wherein the second insulation film comprises an organicinsulator.
 3. The mesa semiconductor device of claim 2, wherein theorganic insulator comprises a polyimide or an epoxy resin.
 4. The mesasemiconductor device of claim 1, wherein the mesa groove extends beyondthe PN junction to reach the first semiconductor layer.
 5. The mesasemiconductor device of claim 1, wherein the first semiconductor layercomprises a first layer and a second layer formed on a front surface ofthe first layer and having a lower concentration than the first layer,and the second layer is in contact with the second semiconductor layer.6. The mesa semiconductor device of claim 5, wherein the mesa grooveextends beyond the PN junction to reach the first layer of the firstsemiconductor layer.